"Very sadly last weekend our dear dog, Lupo, passed away,” the couple wrote in an Instagram post.
“He has been at the heart of our family for the past nine years and we will miss him so much. - W & C."
Fans were quick to share their condolences with the couple.
"I am so so sorry!" one follower wrote. "Losing a dog is so painful. Love to you all."
"Oh, my heart!" another said. "So very sorry for your loss. Dogs are the most precious creatures."
"Oh I remember when Lupo was a pup!" someone else added. "Those furry family members mean so much and I’m so sad to hear of your loss."
Lupo featured in many photos of the royals, including one of the first photos taken of Prince George after he was born.
The English cocker spaniel was a wedding present to Will and Kate in 2011 and was bred from a dog owned by the Duchess' parents Michael and Carole Middleton.
Lupo is even believed to have helped the couple choose George's name.
The pair is rumoured to have written names on paper and scattered them on the floor with Lupo stopping only at George's name.
He even scored a mention after George's birth in 2013.
"For me Catherine and now little George are my priorities, and Lupo," Prince William told CNN.
When asked how Lupo was reacting to the new addition, William continued, "He's coping alright, as a lot of people know who've got dogs and bringing a newborn back, they take a little bit of time to adapt, but he's been alright so far, he's been slobbering around the house, so he's perfectly happy."
Kate's brother James Middleton also shared a tribute to the dog on Instagram, writing, "It is with great sadness that Lupo, the beloved dog of my sister Catherine and her family has passed away. Lupo was the son of Ella, brother to Zulu, Inka & Luna. He was 9 years old.
"Nothing can ever prepare you for the loss of a dog. For those who have never had a dog, it might be hard to understand the loss. However for those who have loved a dog know the truth: a dog is not just a pet; it is a member of the family, a best friend, a loyal companion, a teacher and a therapist."
He continued, "Many people over the years have reached out to me about the pain of [losing] a dog and for most, the loss of a dog is, in almost every way comparable to the loss of a Human loved one. The pain I felt from [losing] my first dog Tilly back in 2017 still hurts and on hearing the news about Lupo brought a wave of emotions flooding back.
"There isn’t much of a rule book on how to grieve for a dog, but I’ve said a prayer, lit a candle and taken Ella (Mum) for a long walk to spend time remembering Lupo.
"Rest in peace Lupo. Tilly & Mini will be waiting for you. You will always be remembered and your legacy will live on forever. Good Boy."
James is a dog lover and even describes himself on Instagram as, "Dog Dad to Ella, Zulu, Inka, Luna, Mabel & Nala."
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